This invention relates to radiation therapy equipment for the treatment of tumors or the like and specifically to an improved method of characterizing the radiation beam of such systems and confirming the dose received by the patient using a portal image of radiation exiting the patient.
Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal-source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal-source radiation therapy has the disadvantages of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as 60Co, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. External-source radiation therapy avoids some of the problems of internal-source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam along with the tumorous tissue.
The adverse effect of irradiating of healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the external radiation beam into the patient at a variety of “gantry” angles with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam thus change as the gantry rotates around the subject, reducing the total dose to each such element of healthy tissue during the entire treatment.
The irradiation of healthy tissue also may be reduced by tightly collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Numerous systems exist for producing such a circumferential collimation, some of which use multiple sliding shutters which, piecewise, may generate a radio-opaque mask of arbitrary outline.
The radiation beam may also be controlled by insertion of wedges or blocks into the beam to reduce the intensity or fluence of the beam by means of attenuation in some areas. U.S. Pat. No. 5,317,616 issued May 31, 1994, incorporated by reference and assigned to the same assignee as the present invention, describes a shutter system that provides an alternative to wedges for reducing fluence or intensity portions of the radiation beam by temporally changing the radiation beam with collimator leaves.
In order to confirm the positioning of the collimation blades and blocks used in a particular radiation therapy session, a “portal image” may be obtained in which treatment radiation exiting from the patient is recorded on x-ray film or the like. A visual examination of this image provides a radiograph that gives a gross indication that the “geometry” of the radiation beam is correct.
Electronic portal imaging devices (EPIDs) are becoming standard features on medical linear accelerators for radiotherapy. These devices enable one to form an anatomical x-ray image of a patient using the high energy treatment beam (average photon energy approximately 2 MeV). Image quality, however, is significantly worse than that routinely achieved in low energy diagnostic x-ray imaging devices for two main reasons, both related to energies of the photons in the beam.
First, most of the contrast in a low energy diagnostic x-ray image comes from the photoelectric effect. Photoelectric absorption of the x-ray photons is proportional to the cube of the atomic number of the absorber. This is why bone (Z=12.5) and soft tissue (Z=7.4) are easily differentiated in a diagnostic x-ray image [(12.5/7.4)^3=5]. Photons with energies above approximately 50 keV interact primarily via Compton scattering, which is independent of atomic number. The rate of Compton interaction of high energy x-rays is simply proportional to electron density. Bone (approx. 5E29 electrons/cc) and soft tissue (approx. 3E29 electrons/cc) are therefore discernable in high energy x-ray images, but contrast is much poorer.
The second problem with high energy x-ray imaging is the nature of Compton interactions versus photoelectric interactions. In a low energy photon interacting via the photoelectric effect the photon is completely absorbed by an electron and does not reach the detector. High energy photons undergoing Compton interactions are not absorbed, however, but instead, they are scattered through some angle. These scattered photons reach the detector and contribute to an image ‘fog’ that further obscures the (already marginal) image detail.
The treatment beams from medical linear accelerators are not monoenergetic, but instead contain an entire spectrum of photon energies ranging from zero up to the nominal energy of the beam (e.g. 6 MeV). Therefore there is, in principle, a good quality image formed by low energy photons in the beam, but it is entirely obscured by the low contrast and scatter contributed by the much more prevalent high energy photons.